Dual tracer resorption and apposition in a rat fracture model

The simultaneous measurement of tracer resorption and apposition in a rat fracture model was accomplished with 99mTc labeled methylene diphosphonate (99mTc-MDP) and 125I labeled phenylphosphonic acid (125I-phi PA), respectively. Loss of 125I-phi PA from the fracture site showed a biexponential release pattern; I = 31% (t1/2b = 2.0 d) and II = 69% (t1/2b = 55.4 d). 99mTc MDP temporal apposition at the fracture site followed the law of allometric growth (y = bxk) with k = 7.75, 10 days post injury. The 125I-phi PA loss differed significantly (P less than 0.001) when compared to normal bone which exhibits a monoexponential release with a t1/2b = 962 days. In the rat tibia fracture model investigated, bone apposition significantly (P less than 0.001) dominates over bone resorption as a function of time. This overwhelming difference partially accounts for the healing process. The apposition-resorption comparison may be a useful index for following the course of skeletal pathology for a variety of model systems.